1. Field of the Invention
The present invention relates generally to the field of iron-binding glycoproteins. More specifically, the present invention relates to the recombinant production of various lactoferrins.
2. Description of the Prior Art
Lactoferrin (LF) is an iron-binding glycoprotein found in milk and other secretions and body fluids. It is one of a number of iron binding proteins, sometimes referred to as transferring, and is involved in iron binding and delivery in mammals.
Human lactoferrin (hLF) is a member of the transferrin family of iron-binding monomeric glycoproteins. It was originally discovered in milk where it can reach levels of 7 grams/liter in colostrum. LF has since been detected in other external fluids of humans and other mammals. The fluids include tears, saliva and mucosal secretions and also in the secondary granules of polymorphonuclear leukocytes.
Lactoferrin has been implicated as a factor in resistance against enteritis infections in suckled newborn humans. The bacteriocidal/bacteriostatic actions are considered to be due at least in part to the iron binding properties of lactoferrin. Lactoferrin decreases the iron availability to iron-requiring microorganisms and thereby interferes with their growth and reproduction. At least one non-iron-binding bactericidal domain has also been reported for human lactoferrin. Lactoferrin is also considered to have antiviral properties and to have other potential therapeutic applications.
LF is a 78 kilo Dalton (k Da) glycoprotein having a bilobal structure with a high degree of homology between the C and N terminal halves which is evident at both the amino acid and three dimensional structural level. Each of these lobes can reversibly bind one ferric iron with high affinity and with the concomitant binding of bicarbonate. The biological functions proposed for lactoferrin include protection against microbial infection, enhanced intestinal iron absorption in infants, promotion of cell growth, regulation of myelopoiesis and modulation of inflammatory responses.
Human lactoferrin (hLF) has a high affinity for iron and two Fe3+ cations can be bound per molecule. The complete HLF protein has been subjected to amino acid sequencing and is reported to have 703 amino acids. There are two glycosylation sites. Metz-Boutigue et al., Eur. J. Biochem., 145:659-676 (1984). Anderson et al., Proc. Nat""l Acad. Sci. USA, 84:1769-1773 (April 1987).
In other studies, a cloned cDNA probe for amino acids 428 to 703 of the Metz-Boutigue structure of the lactoferrin protein was isolated. The cDNA sequence was in general agreement with the earlier analysis of the amino acid sequence of the protein. Rado et al., Blood, 79; 4:989-993, 79; 4:989-993 (October 1987). The probe was reported to encompass approximately 40% of the coding region and the 3xe2x80x2 terminus. The cDNA sequence for both porcine, Lydon, J. P., et al., Biochem. Biophysic. ACTA, 1132:97-99 (1992); Alexander, L. J., et al., Animal Genetics, 23:251-256 (1992) and bovine lactoferrin, Mead, P. E., et al., Nucleic Acids Research, 18:7167 (1990); Pierce, A., et al., Eur. J. Biochem., 196:177-184 (1991), have been determined.
Polypeptides derived from lactoferrin are also known to be biologically active. A fragment containing a possible iron binding site was reported by Rado, et al. supra. An N-terminal human lactoferrin fragment, including a bactericidal domain of HLF, was isolated from a pepsin digest. Bellamy, W. M., et al., Biochem. Biophys. ACTA, 1121:130-136 (1992). Synthetic 23 and 25 amino acid polypeptides were synthesized and found to have activities similar to the fragments derived by pepsin digestion. The synthesis details, yields and purity of the synthetic peptides were not reported. Bellamy et al. do not provide a practical route to large scale production of the polypeptides free of the contaminates resulting form isolation from natural products.
The bactericidal domain from lactoferrin has a broad spectrum of antimicrobial action. Bellamy, W. M. et al., J. App. Bact. 73, 472-479 (1992). Although Bellamy et al. report that bovine lactoferrin isolated from milk can provide commercial quantities of the bovine polypeptide by pepsin digestion, the materials used in both studies had a minimum purity of only 95%. Bellamy, et al. do not provide constructs for the large scale production of synthetic human or bovine lactoferrin or lactoferrin polypeptides. Neither does Bellamy et al. provide the ability to produce peptides that are not available by enzyme digestion.
Filamentous fungi have been successfully employed as hosts in the industrial production of extracellular glycoproteins. Certain industrial strains are capable of secreting gram quantities of these proteins. In addition, filamentous fungi are able to correctly perform post-translational modifications of eucaryotic proteins and many strains have U.S. Food and Drug Administration approval. Furthermore, large scale fermentation technology and downstream processing experience is available.
Currently, there is no efficient and economical way to produce hLF, other species lactoferrin, or to control production of lactoferrin polypeptides. Consequently, a long felt need and description in this art would be met by the development of an efficient method for the production of human lactoferrin for nutritional and therapeutic applications and for further investigation into its mechanism of action.
The invention comprises the verified cDNA sequences for human lactoferrin, and cDNA expression systems for use of various lactoferrin DNA sequences to produce human, bovine, porcine and other lactoferrins for a variety of end uses. The cDNA expression systems of the invention also provide a practical route and method to make lactoferrin polypeptides or fragments having biological activity. The hLF cDNA includes an open reading frame of 2133 nucleotides coding for a protein of 711 amino acids. These 711 amino acids include 19 amino acids corresponding to a secretion signal peptide sequence followed by 692 amino acids of mature human lactoferrin. The cDNA sequence and deduced amino acid sequence differ from the previously published data of Metz-Boutigue, supra.
In one embodiment, the present invention provides for a recombinant plasmid comprising the cDNA of human or other lactoferrin. The plasmid of the present invention is adapted for expression in a eucaryotic cell and contains the regulatory elements necessary for the expression of the human lactoferrin cDNA in this eucaryotic cell.
In another embodiment, the present invention provides for a transformed cell which includes a heterologous DNA sequence which codes for lactoferrin or a polypeptide related to lactoferrin. The heterologous DNA sequence will preferably be incorporated into a plasmid. Eucaryotic host cells are selected from the group consisting of mammalian cells, immortalized mamunalian cells, fungi or yeasts. Preferred cells include filamentous fungi comprising Aspergillus, and yeasts. The plasmid contains a plasmid vector into which a polydeoxyribonucleotide (DNA) segment coding for human or other lactoferrin protein has been inserted.
In yet another embodiment of the present invention, there is provided a process for producing recombinant human or other lactoferrin which comprises culturing a transformant eucaryotic cell, which includes a recombinant plasmid. The plasmid contains a plasmid vector having a polydeoxyribonucleotide coding for the lactoferrin protein. After culturing in a suitable nutrient medium until lactoferrin protein is formed, the lactoferrin protein is isolated.
In still yet another embodiment of the present invention, there is provided a recombinant expression vector. This vector comprises a transcriptional unit comprising an assembly of (1) a genetic element or elements having a regulatory role in gene expression; (2) cDNA coding for lactoferrin; (3) appropriate transcription and translation initiation and termination sequences; and (4) a genetic element for selection of transformed cells or spores such as Aspergillus spores that have been transformed with the vector.
In still yet another embodiment of the present invention, there is provided a method for producing biologically active recombinant lactoferrin. The method comprises synthesizing sequences containing a selectable marker gene, a promotor, a transcription termination sequence, and a linker sequence; cloning the sequences to form a plasmid; digesting the plasmid with a restriction endonuclease; inserting a cDNA coding for lactoferrin into a restriction site; and transforming eucaryotic cells with the plasmid expressing lactoferrin cDNA.